A common power control approach in interference-limited communication systems relies on a receiver feeding back transmit power control commands to a transmitter. The commands streaming back from the receiver tell the transmitter to incrementally increase or decrease its transmit power on an ongoing basis, as needed to maintain some reception metric for the transmitter's signal as received by the receiver. Received signal quality, expressed as a signal-to-noise ratio (SNR) or signal-to-interference-plus-noise ratio (SINR), is a common reception metric.
As with many aspects of wireless communication system operations, power control is becoming increasingly complicated with increasing data rates. For example, Wideband Code Division Multiple Access (WCDMA) was originally developed for circuit voice and moderate data rates, employing long (10 ms) Transmission Time Intervals (TTIs). Uplink transmissions always include the Dedicated Physical Control Channel (DPCCH)—which is a fixed-rate control channel—thereby providing a reference for SINR-based closed-loop power control. When the UE has traffic data to send, the NodeB (a WCDMA base station) grants the UE a transmit power allocation on the Enhanced Dedicated Physical Data Channel (E-DPDCH) that is relative to the DPCCH power.
Doing so is synonymous with granting a data rate to the UE for its uplink transmission, as there is a fixed table relating relative power and data rate sent to the UE at set-up. As data is transmitted, outer-loop power control is employed, with the NodeB raising or lowering the target SINR value for receiving the E-DPDCH from the UE, depending on whether block errors occur. The NodeB carries on such power control to maintain a target block-error-rate (BLER) for the traffic data incoming from the UE.
The above-described approach to uplink power control rests on a number of assumptions that were true when WCDMA systems were first deployed. First, the SINR-based closed-loop power control approach assumes that SINR can change within a TTI, which is true when the TTI is long (10 ms) relative to the fading rate. Second, such a closed-loop power control assumes that the NodeB has enough excess resources (received power headroom) to allow a UE to use more resources (increase its received power). Third, the data rate selection based on a fixed relation between power and rate assumes that self-interference is not significant. This third assumption holds true at moderate data rates.
However, the WCDMA uplink has evolved to a point where these assumptions no longer hold. As for the first assumption, a shorter TTI (2 ms) has been introduced so that signal quality is approximately constant over a TTI. As for the second and third assumptions, data rates have become high enough that self-interference is significant, even after equalization. As a result, SINR does not simply scale with signal power S, but also depends on a fading-realization-dependant orthogonality factor. Consequently, there is a channel-dependent relation between power and supportable rate, and instability can result when the target SINR value is above the SINR ceiling.
Known approaches to one or more of the above problems include adapting closed-loop power control based on a measure of S/(I+N) instead of S/(self I+I+N)—see, e.g., WO 2008/057018 (published on 15 May 2008). As noted, S is signal power (received), and I is co-channel interference from own-cell and other-cell signals, N is thermal noise, and “self I” is the self interference due to dispersive channels. This approach reduces instability at the expense of performance (block error rate increases). As a result, more retransmissions occur, thus increasing delay (latency).
In another alternative, a series of power commands are inhibited to improve stability—see, e.g., U.S. utility patent application Ser. No. 12/022,346, filed on 30 Jan. 2008. As with the above-noted power control adaptation, this approach can degrade performance, thus causing more retransmissions. Yet another alternative introduces a second outer-loop power control loop, so that quality (SINR) of traffic and control data can be just met, rather than one being exceeded. While this approach improves efficiency, it does not address the instability caused when SINR requirements cannot be met.
In another alternative, the power of the traffic relative to the control (traffic gain or beta factor) is adapted to maintain quality (SINR) on the control channel in addition to the SINR of the traffic channel. This is done either at the network side (based on measured quality of the control channel) or the UE side (based on ACK/NACK feedback from the NodeB). This technique, by itself, does not solve the power instability problem, as self interference can cause SINR targets not to be met.